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| CMS-PAS-HIN-25-010 | ||
| First measurement of pseudorapidity distributions of charged hadrons in oxygen-oxygen collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV with CMS | ||
| CMS Collaboration | ||
| 2025-09-08 | ||
| Abstract: We report the first measurement of the charged hadron pseudorapidity ($ \eta $) distributions in oxygen-oxygen collisions at a nucleon-nucleon center-of-mass energy of $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV. The data were recorded by the CMS experiment at the LHC in 2025. The yields of primary charged hadrons produced in the range $ \lvert \eta \rvert < $ 2.4 are reported using the CMS silicon pixel detector. The midrapidity particle density as a function of collision centrality is also reported. In the 5% most central collisions, the charged-hadron $ \eta $ density in the range $ \lvert \eta \rvert < $ 0.5 is found to be 135 $ \pm $ 3 (syst), with negligible statistical uncertainty. The data are compared to previous measurements of lead-lead and xenon-xenon collisions at similar collision energies and several Monte Carlo event generators. Detailed studies of the dependence of particle production on the collision energy, initial collision geometry, and the size of the colliding nuclei are presented. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ distributions in OO collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV for events in the 0--100% centrality class. The results have been averaged and symmetrized around $ \eta= $ 0. Predictions from the AMPT 1.26t5 [38], EPOS LHC v3400 [34,21], PYTHIA8 Angantyr [39] and HIJING [33] event generators are also displayed. The ratios of the $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ distributions of simulation and data, normalized to unity at midrapidity, are shown in the bottom panel. In the upper panel the gray boxes show the total systematic uncertainties and the statistical uncertainties are negligible. In the lower panel, the boxes represent the relative uncertainty of the data. |
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Figure 2:
Charged-hadron $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ in OO collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV at midrapidity as a function of event centrality, shown as is (left) and normalized by 2 $ A $ (right), where $ A $ is the atomic number of the nuclei. The results are compared to measurements in PbPb and XeXe collisions at similar collision energy by the CMS [13,18] and ALICE [17,14,19] Collaborations. The bands around the data points denote the total systematic uncertainties, while the statistical uncertainties are negligible. |
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Figure 2-a:
Charged-hadron $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ in OO collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV at midrapidity as a function of event centrality, shown as is (left) and normalized by 2 $ A $ (right), where $ A $ is the atomic number of the nuclei. The results are compared to measurements in PbPb and XeXe collisions at similar collision energy by the CMS [13,18] and ALICE [17,14,19] Collaborations. The bands around the data points denote the total systematic uncertainties, while the statistical uncertainties are negligible. |
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Figure 2-b:
Charged-hadron $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ in OO collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV at midrapidity as a function of event centrality, shown as is (left) and normalized by 2 $ A $ (right), where $ A $ is the atomic number of the nuclei. The results are compared to measurements in PbPb and XeXe collisions at similar collision energy by the CMS [13,18] and ALICE [17,14,19] Collaborations. The bands around the data points denote the total systematic uncertainties, while the statistical uncertainties are negligible. |
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Figure 3:
Average $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity normalized by $ \langle N_{\text{part}} \rangle $, shown as a function of $ \langle N_{\text{part}} \rangle $ (left) and $ \langle N_{\text{part}} \rangle/2A $ (right), where $ A $ is the atomic number of the nuclei. The results are compared to measurements in PbPb and XeXe collisions by the CMS [18,13] and ALICE [19,17,14] Collaborations. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible. |
png pdf |
Figure 3-a:
Average $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity normalized by $ \langle N_{\text{part}} \rangle $, shown as a function of $ \langle N_{\text{part}} \rangle $ (left) and $ \langle N_{\text{part}} \rangle/2A $ (right), where $ A $ is the atomic number of the nuclei. The results are compared to measurements in PbPb and XeXe collisions by the CMS [18,13] and ALICE [19,17,14] Collaborations. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible. |
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Figure 3-b:
Average $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity normalized by $ \langle N_{\text{part}} \rangle $, shown as a function of $ \langle N_{\text{part}} \rangle $ (left) and $ \langle N_{\text{part}} \rangle/2A $ (right), where $ A $ is the atomic number of the nuclei. The results are compared to measurements in PbPb and XeXe collisions by the CMS [18,13] and ALICE [19,17,14] Collaborations. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible. |
png pdf |
Figure 4:
Average $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity, shown as a function of event centrality (left) and $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ normalized by $ \langle N_{\text{part}} \rangle $ as a function of $ \langle N_{\text{part}} \rangle $ (right). The results are compared to predictions from the AMPT 1.26t5 [38], EPOS LHC v3400 [34,21], PYTHIA8 Angantyr [39] and HIJING [33] event generators, as well as a hydrodynamic model $ Trajectum $ [43]. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible. |
png pdf |
Figure 4-a:
Average $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity, shown as a function of event centrality (left) and $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ normalized by $ \langle N_{\text{part}} \rangle $ as a function of $ \langle N_{\text{part}} \rangle $ (right). The results are compared to predictions from the AMPT 1.26t5 [38], EPOS LHC v3400 [34,21], PYTHIA8 Angantyr [39] and HIJING [33] event generators, as well as a hydrodynamic model $ Trajectum $ [43]. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible. |
png pdf |
Figure 4-b:
Average $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity, shown as a function of event centrality (left) and $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ normalized by $ \langle N_{\text{part}} \rangle $ as a function of $ \langle N_{\text{part}} \rangle $ (right). The results are compared to predictions from the AMPT 1.26t5 [38], EPOS LHC v3400 [34,21], PYTHIA8 Angantyr [39] and HIJING [33] event generators, as well as a hydrodynamic model $ Trajectum $ [43]. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible. |
png pdf |
Figure 5:
Comparison of average $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity, scaled by $ \langle N_{\text{part}} \rangle $ in $ \mathrm{p} $Pb [44,28], pAu [45], dAu [46,47,48] (pA), and central heavy ion (AA) collisions [16,18,13,49,50,51,52,12,53,54,55,56,57,19,47,58,59], as well as non-single-diffractive (NSD) [23,24,59,60,61,62] and inelastic [27,49,63,64] pp collisions. The data points for AA collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 2.76 TeV from ALICE and ATLAS have been shifted horizontally by $ \pm 10% $ for visibility. The dashed curves, reproduced from Ref. [28], are included to guide the eye, and correspond to a power law functional form. |
| Tables | |
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Table 1:
Centrality intervals and corresponding $ \langle N_{\text{part}} \rangle $ values for 5.36 TeV OO collisions. The uncertainties in the $ N_{\text{part}} $ values are determined by using a variety of oxygen profiles in the Glauber calculation, and by varying the reference point as done for the data. |
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Table 2:
Sources of systematic uncertainty affecting the measurement of charged-hadron multiplicities as a function of $ \eta $ and centrality classes in OO collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV. |
| Summary |
| The pseudorapidity ($ \eta $) distributions of charged hadrons are measured in the range $ |\eta| < $ 2.4 for multiple centrality intervals using data collected in oxygen-oxygen (OO) collisions for the first time at the center-of-mass energy of $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV. The dependence on $ \eta $ is compared to the event generators HIJING, EPOS LHC v3400, PYTHIA8 Angantyr, and AMPT 1.26t5. Among the models tested, AMPT reproduces the midrapidity normalization, while EPOS LHC and Angantyr best capture the $ \eta $-shape once normalized at $ \eta= $ 0; however, Angantyr overpredicts and EPOS LHC underpredicts the absolute yields. The midrapidity results are compared with previous measurements in various collision systems including PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 and 5.36 TeV and XeXe collisions at 5.44 TeV. In comparison, we observe the expected system-size ordering--$ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ increases from OO to XeXe to PbPb at fixed centrality. After normalization by 2 $ A $, OO follows the trend only approximately and exhibits a shallower centrality dependence. At a given participant fraction, per-participant yields roughly agree between PbPb and XeXe, but OO shows a significant deviation. Together, these results still exhibit an AA energy-scaling systematics while reveal a breakdown of previously observed participant and 2 $ A $ scalings in the smallest ion-ion system, underscoring the role of initial geometry and small-system dynamics. For the 5% most central collisions, the charged-hadron density, $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $, for the range $ |\eta| < $ 0.5 is found to be 135 $ \pm $ 3 (syst), with negligible statistical uncertainty. Per participant $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity in the centrality class of 0--5% in OO is found to be consistent with that of lead-lead collisions and to be larger than that of pp collisions. The OO results provide new, system-specific constraints on theoretical models and event generators of multiparticle production in relativistic heavy-ion collisions. |
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